Preparation And Performance Of Ca-based Compound Oxygen Carriers Used In Chemical-looping Combustion Of Coal

Coal directed chemical-looping combustion (CD-CLC) is an attractive technology.Coal is directly mixed with oxygen carriers with the steam used as the gasificationmedium，and the syngas is reacted with oxygen carriers. Oxygen carrier preparation isthe keystage to its industrial application. In this thesis, some additions and inertmaterialswere introduced to prepare the Ca-based compound oxygen carriers and thecharacteristic of the oxygen carriers was investigated in a fluidized bedwith thesteamused as the gasification-fluidization medium. The main results are summarized asfollows:(1) Ca-based compound oxygen carriers were prepared with industrial gradematerials natural anhydrite, bentonite, using mechanical mixing-prilling method. Itshowed that the optimum mass content of CaSO4was60%. The attrition rate was0.089%·h-1. The reactivity of the oxygen carriers was enhanced as temperature wasincreased from800°C to900°C. At900°C, the average dry basis concentration ofCO2approached89.52%. The carbon conversion efficiency was above70%and theCO2concentration maintained at80%over seven redox cycles. X-ray diffractionanalysis revealed that CaSO4was reduced into CaS without generating CaO.(2) The Fe2O3component was added by impregnating the oxygen carrier particalsinto Fe(NO3)3solution to improve their reactivities. The time reaching a carbonconversion efficiency of95%was shortened to20.8min, and the average dry basisconcentration of CO2approached95.99%. Ten redox tests demonstrated that theconcentration of CO2kept stable at80%and the CaSO4-Fe2O3/Bentonite oxygencarrier particles were featured by the high reactivity and excellent cycling stability.Moreover, four types of coal were used, where the coals with higher volatile contentand higher ash content showed higher combustion efficiencies. The CO2concentration of the gasification products was above90%in all the cases. Particle size distributioncurves indicated high attrition resistance of CaSO4-Fe2O3/Bentonit.(3) The K2CO3was added by impregnating CaSO4/Bentonite into K2CO3solutionto enhance its reactivity. The CO2generationrate reached a maximum of0.45L·min-1at4min. The time needed to reach a carbon conversion efficiency of95%wasshortened to13min, and the average dry basis concentration of CO2approached85%.The oxygen carrier obtained porous structure during the first5cycles. However, theoxygen carrier particles were seriously sintered and the loss of potassium on thesurface appeared after redox cycles. The coal conversion rate was increasedsignificantly because catalytic gasification occurred between K2CO3and three types ofcoal.